South Korea conducted maritime surveys last year near a group of islets at the center of a territorial dispute with Japan for the first time in a decade, a Japan Coast Guard official said Thursday.

Despite the Japan Coast Guard's call to stop the activities, South Korea went ahead with the surveys twice last year in waters claimed by Tokyo as part of its exclusive economic zone near the Seoul-controlled islets in the Sea of Japan, the official said, in another mark of tensions between the two neighbors.

No such survey had been conducted by South Korea or Japan since their bilateral relationship deteriorated in 2006 after Japan announced plans to survey an area near the islets. At the time, South Korea protested by announcing its own research mission.

According to the official, a Russian research vessel conducted a study for Seoul National University some 38 km west of the islets on April 9 last year, while on June 18, a South Korean research vessel was spotted with observation equipment around 80 km southwest of the islets.

The islets, which Japan calls Takeshima, are known as Dokdo in South Korea.

On top of the territorial dispute, the relationship remains strained over the issue of "comfort women," who were forced to work in Japanese wartime military brothels.

Japan has said statues symbolizing the women near Japanese diplomatic missions in South Korea, including a new one in December, conflict with the spirit of a 2015 bilateral agreement to "finally and irreversibly" resolve the dispute.